“Brake-by-wire” brake systems are being used to an ever-increasing extent in motor vehicle engineering. In addition to a brake master cylinder that can be actuated by the vehicle driver, brake systems of this kind often include a pressure supply device which can be controlled electrically (“by-wire”) and by means of which actuation of the wheel brakes is performed in the “brake-by-wire” operating mode.
In these brake systems, in particular electrohydraulic brake systems featuring the “brake-by-wire” operating mode, the driver is isolated from direct access to the brakes. When the pedal is actuated, a pedal decoupling unit and a simulator are usually actuated, wherein the braking requirement of the driver is detected by a sensor system. The pedal simulator, which is usually designed as a brake master cylinder, serves to give the driver a brake pedal feel which is as familiar and comfortable as possible. The brake requirement detected leads to a determination of a setpoint braking torque, from which the setpoint brake pressure for the brakes is then determined. The brake pressure is then built up actively in the brakes by a pressure supply device. The actual braking is thus accomplished by active pressure buildup in the brake circuits with the aid of a pressure supply device, which is controlled by an open-loop and closed-loop control unit. By means of the hydraulic decoupling of brake pedal actuation from the pressure buildup, many functionalities, such as ABS, ESC, TCS, hill start assistance etc. can be implemented in a manner that is convenient for the driver in brake systems of this kind.
In brake systems of this kind, a hydraulic fallback level is usually provided, by means of which the driver can brake the vehicle or bring it to a halt by muscle power when the brake pedal is actuated if the “by-wire” operating mode fails or is faulty. Whereas the above-described hydraulic decoupling between brake pedal actuation and brake pressure buildup is performed by a pedal decoupling unit in the normal mode, this decoupling is canceled at the fallback level, thus allowing the driver to move brake fluid into the brake circuits directly.
The pressure supply device in brake systems described above is also referred to as an actuator or hydraulic actuator. In particular, actuators are designed as linear actuators or linear units, in which a piston is moved axially into a hydraulic pressure chamber installed in series with a rotation-translation mechanism in order to build up pressure.
WO 2011/029812 A1 discloses a “brake-by-wire” brake system having a brake master cylinder that can be actuated by means of a brake pedal, a travel simulator and a pressure supply device. In a “brake-by-wire” operating mode, the wheel brakes are supplied with pressure by the pressure supply device. In a fallback operating mode, the wheel brakes are supplied with pressure by means of the brake master cylinder, which can be actuated by means of the brake pedal.
To draw in additional brake fluid in known brake systems, the pressure supply device is usually connected to a reservoir by a hydraulic line, into which a check valve is inserted. The suction side of the check valve is hydraulically connected to the reservoir, and the blocking side of the check valve is hydraulically connected to the pressure supply device.
A switch is made to the fallback level if a pressure buildup is no longer possible with the aid of the pressure supply device. This is the case, inter alia, when the check valve connecting the pressure supply device to the reservoir no longer shuts off reliably, with the result that a pressure buildup is no longer reliably possible.
During automated driving, in which the vehicle drives autonomously, it must be assumed that there are long periods when the driver is not in a position to perform braking at the fallback level if this check valve fails.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.